A basic three-mirror anastigmat optical system uses a positive-optical-power primary mirror, a negative-optical-power secondary mirror, and a positive-optical power tertiary mirror arranged along a beam path. The primary mirror and the secondary mirror form an intermediate image that is relayed to a final image plane by the tertiary mirror. The structure and function of the three-mirror anastigmat optical system are discussed more fully in U.S. Pat. No. 4,265,510, whose disclosure is incorporated by reference.
It is desirable for some applications that the telescope be compact, with a ratio of the physical length to the effective focal length (PL:EFL) of the telescope that is small. A compact telescope occupies a relatively small package, with the associated small size and light weight. The small size and light weight are desirable for telescopes that are to be launched into space, to meet booster envelope and weight limitations. The conventional three-mirror anastigmat works well for many such applications requiring a compact telescope. For example, for a fast, short-focal-length optical system with an optical speed in the range of F/2.5-F/3, the PL:EFL ratio may be made to be about 0.5, judged sufficiently compact for this fast optical speed
However, for other applications, the three-mirror anastigmat cannot readily be built in a sufficiently compact arrangement without highly undesirable optical and manufacturing consequences. For example, when the optical speed is in the F/5 to F/6 range, for some applications it is desirable that the PL:EFL ratio be less than about 0.25-0.30, which cannot readily be achieved with the three-mirror anastigmat.
There is a need for a design of an all-reflective, relayed optical-system with the telescope characteristics of the three-mirror anastigmat but with a substantially smaller ratio of PL:EFL than possible with the known three-mirror anastigmat. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system in the form of a compact four-mirror anastigmat telescope. The optical system is all-reflective, flat field, relayed, and with an unobscured aperture. It has a small ratio of physical length to effective focal length in the medium optical speed range, producing a relatively short, relatively light weight telescope. The optical system of the invention maintains good optical performance in respect to low aberrations, low distortion, reasonable pupil aberrations, and good focal plane incidence angles, as found in conventional three-mirror anastigmatic optical systems.
In accordance with the invention, an optical system comprises a three-mirror anastigmat including a primary mirror, a secondary mirror, and a tertiary mirror positioned to reflect a beam path. An intermediate image is formed on the beam path at an intermediate-image location between the secondary mirror and the tertiary mirror. A negative-optical-power field mirror is positioned in the beam path at a field-mirror location subsequent to the intermediate-image location along the beam path. The field mirror reflects the intermediate image to the tertiary mirror.
In the preferred design, the primary mirror and the tertiary mirror each have positive optical power, and the secondary mirror has negative optical power. The field mirror has negative optical power, preferably such that a ratio of a field mirror focal length to a secondary mirror focal length is between about 1.25 and about 1.75. To obtain a flat field, the sum of the optical powers of the primary mirror, the secondary mirror, the field mirror, and the tertiary mirror is preferably substantially zero. In one example, the primary mirror is a primary-mirror ellipsoid, the secondary mirror is a secondary-mirror hyperboloid, the tertiary mirror is a tertiary-mirror ellipsoid, and the field mirror is a field-mirror sphere.
In one embodiment, the optical speed of the optical system is from about F/5 to about F/6. A ratio of physical length to effective focal length of the optical system is less than about 0.25.
Stated in other terms, an optical system comprises a positive-power primary mirror, a negative-power secondary mirror, a negative-power field mirror, and a positive-power tertiary mirror. The mirrors are arranged such that a beam path is reflected from the primary mirror to the secondary mirror to the field mirror to the tertiary mirror, and an intermediate image is formed in the beam path after reflection from the secondary mirror but before reflection from the field mirror. The intermediate image is reflected by the field mirror to the tertiary mirror and thence to an image location. Other features discussed herein may be used with this embodiment.
In another embodiment, an optical system comprises a set of four powered mirrors whose powers sum to substantially zero and which are arranged such that a beam path is reflected from a primary mirror to a secondary mirror to a field mirror to a tertiary mirror to an image plane. An intermediate image is formed in the beam path after reflection from the secondary mirror. The intermediate image is reflected by the field mirror to the tertiary mirror and thence to the image plane. An optical speed of the optical system is from about F/5 to about F/6, and a ratio of physical length to effective focal length of the optical system is less than about 0.25. Other features discussed herein may be used with this embodiment.
The present approach utilizes the features of the three-mirror anastigmat, and then adds the field mirror to reduce the physical length of the optical system relative to the effective focal length. The optical system is thereby reduced in size and weight, important advantages in applications such as space-based optical systems. The quality of the image is retained to a high degree. The optical system is an all-reflective type, which avoids chromatic aberrations introduced when lenses are used. The present all-reflective approach is therefore more suitable for imaging a light beam with a wide range of wavelengths, as compared with a refractive optical system.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.